The basic technique in forming electrodeposited foil has not changed greatly over the years. In this respect, electrodeposited copper foil is generally formed by immersing a rotating drum cathode in an electrolytic solution containing copper ions. An anode formed from one or more arcuate sections of electrically conductive material is immersed in the electrolytic solution and positioned adjacent the drum cathode. The anode is formed to have a surface generally conforming to the curvature of the drum cathode so as to define a uniform inner electrode gap therebetween. Copper foil is formed on the rotating drum by applying a current, having a current density lower than the limiting current density of the electrolytic solution, to the anode and cathode. The electrodeposited foil is continually removed from the drum cathode as it emerges from the electrolytic solution so as to permit continuous foil production.
The actual production of copper foil begins through the nucleation of copper atoms on the surface of the drum cathode as the cathode initially enters the electrolytic solution. Copper continuously builds up onto these copper atoms as the drum continues to rotate past the energized anode through the electrolytic solution.
The current distribution at the entry point of the drum cathode surface has a significant effect on the quality of the copper nucleation. In this respect, copper nucleation is a rapid process, and it has been found that a uniform, sharp rise in current density at the point where the surface of the drum cathode enters the electrolytic solution can remarkably increase the density of copper formed on the drum, which in turn, reduces the porosity of the resulting foil. In other words, a quick current rise at the surface of the drum as it enters the electrolytic solution is critical for good copper nucleation. As the demand for thinner foils becomes greater, the initial copper nucleation on the drum surface becomes more important to insure porous-free copper foil.
Conventional electrolytic cells known heretofore have typically included anodes that were totally immersed in the electrolytic solution. Such arrangements produce slow current "ramping-up" as the drum cathode enters the electrolytic solution, which in turn, causes insufficient copper nucleation on the surface of the drum cathode. This slow current ramp-up occurs because the desired current density on the surface of the drum cathode does not occur until the surface of the drum is radially opposite the immersed anode. To improve the current ramping-up time, it has been known to place an insulator shield on the top (i.e., along the upper edge) of the immersed anode. While such an arrangement improves the current ramp up compared to an immersed anode alone, it does not completely eliminate the problem. To further improve the copper nucleation, it has also been known to use a strike anode (second anode) disposed near the surface of the electrolytic solution at the entry point of the surface of the drum cathode. The strike anode is energized at a higher current density than the main anode. A problem with this arrangement is that it requires a second rectifier to control the second anode, i.e., the strike anode. Moreover, though copper nucleation is improved using a strike anode, such a method does not totally eliminate the slow current ramping-up problem.
U.S. Pat. No. 5,833,819 to O'Hara et al. proposes the use of a partially immersed "net-type strike anode" instead of a solid strike anode to reduce ramping-up time. While this net-type strike anode significantly reduces current ramping-up time, and improves the copper nucleation, it still requires a second rectifier for operation. Moreover, both the aforementioned "strike anode" and the net-type strike anode disclosed in U.S. Pat. No. 5,833,819 disclose applying a higher current density to the surface of the drum cathode as it enters the electrolytic solution, and both require some type of insulator plate between the strike anode and the main anode.
The present invention overcomes these and other problems and provides an anode that eliminates slow current ramping-up problems, and does not require an insulator plate or a second rectifier for supplying a higher current density to the surface of the drum cathode.